52 Water milfoil (Myriophyllum alterniflorum). Photo: Mps197/Shutterstock.com 53 4.4. Macrophytes 4.4.1. Introduction 4.4.3.Overall Patterns and Trends Macrophytes are a diverse group of aquatic plants large 4.4.3.1. Circumpolar Diversity enough to see with the naked eye. There are 644 described species of vascular macrophytes in the Nearctic region and Among the three ecoregions with the largest number of 497 species in the Palearctic region (Chambers et al. 2008), sampling stations, there was significantly lower alpha diversity though Arctic zones of these biogeographical regions are in the Iceland Boreal Birch Forests and Alpine Tundra (estimate expected to be less diverse. Macrophytes are taxonomically of 35 species at 70 stations) than in either the Scandinavian and phenologically wide ranging, from macroalgae (such and Russian Taiga or the Scandinavian Montane Birch Forest as macroscopic species of green algae or Chlorophyta), to and Grasslands (estimate of 111 and 112 species at 70 mosses and liverworts (Bryophyta), ferns (Pteridophyta) stations, respectively; Figure 4-22b). Macrophyte distribution and seed-bearing plants (Spermatophyta) (Chambers et al. ranges are thought to be largely determined by seed dispersal 2008). Macroscopic forms of Cyanobacteria, Xanthophyta via migratory birds and human activity, though continental (yellow-green algae) and Rhodophyta (red algae) can also drift and geographic proximity may have influenced dispersal be classified as aquatic macrophytes. Morphological forms patterns (Les et al. 2003, Chambers et al. 2008). These of aquatic macrophytes include emergent (rooted plants processes may have contributed to lower observed alpha with foliage extending into the air), floating-leaved (plants diversity in Iceland compared to the continental Scandinavian rooted to the lake or stream bottom with leaves that float on ecoregions. When compared to one another using 100 the water surface), submersed (plants growing completely stations, alpha diversity estimates were similar for the two submerged under the water and attached to, or closely Scandinavian ecoregions (120 species in the Scandinavian associated with the substrate), and free-floating macrophytes and Russian Taiga and 130 species in the Scandinavian (plants that typically float on or under the water surface) Montane Birch Forest and Grasslands), indicating generally (Chambers et al. 2008). In addition, the depth distribution of high diversity within these two ecoregions. macrophytes in lakes and rivers is often determined by the light penetration through water. Rarefaction of species across all ecoregions, using 10 stations as the assessment threshold, showed alpha diversity Macrophytes are an important functional component of estimates were lowest (< 15 species) for the tundra ecoregions lake ecosystems. They remove nutrients (e.g., nitrogen of the Brooks-British Range, Kalaallit Nunaat High Arctic, and phosphorus) from the water column (e.g., Gumbricht Kola Peninsula and High Arctic (Figure 4-22c). Three of the 1993, Jeppesen et al. 1998) and decrease wave energy and ecoregions with the lowest species richness were located water currents, which leads to increased sedimentation at the highest latitudes (average latitude > 70°N for the and stabilization of sediment within macrophyte beds (e.g., stations in the ecoregion), suggesting that alpha diversity of Carpenter and Lodge 1986, Sand-Jensen 1997). Moreover, macrophytes declines in high-latitude Arctic regions. This is these beds provide habitat for fish, invertebrates, and consistent with past research, which has suggested that there epiphytes, and are an important food source for some are latitudinal and altitudinal gradients in alpha diversity invertebrates (e.g., insects) and vertebrates (e.g., fish, birds, (Chambers et al. 2008), with aquatic vascular macrophytes moose) (Lodge 1991, Newman 1991). Wrona et al. (2013) showing a decline in species richness with latitude (Wrona indicate there are several major environmental factors that et al. 2013). The highest alpha diversity (> 45 species) was in affect macrophyte distribution including nutrient levels, lakes of the Arctic Coastal Tundra, Northwest Territories Taiga, water clarity and water temperature (including ice regimes). Scandinavian and Russian Taiga and Scandinavian Montane Because macrophyte presence and abundance is closely Birch Forest and Grasslands (Figure 4-22c). Interestingly, the associated with these environmental factors as well as Scandinavian and Russian taiga ecoregion had the highest substrate type, the composition of macrophyte communities estimated alpha diversity when only 10 samples were used can provide diagnostic information on water quality and is (60 species), whereas it generally had lower alpha diversity part of many countries’ assessment criteria (Jeppesen et al. than the Scandinavian Montane Birch Forest and Grasslands 1998, Søndergaard et al. 2010). when a more representative sample size (e.g., over 70 stations) was considered. This result highlights the importance of 4.4.2. Objectives and Approach sampling a sufficient number of stations across these regions. This circumpolar assessment provides a summary of broad Beta diversity of macrophyte assemblages ranged between spatial patterns of aquatic macrophyte biodiversity in the 0 (no inter-station differences in species composition) and 1 Arctic. To accomplish this we examined presence/absence (no inter-station overlap in species) within the ecoregions. data for macrophyte species-level data compiled for 440 Ecoregions with the highest inter-station differences (βSOR > lakes in all Arctic countries except Russia (Figure 4-22a). We 0.80) included the Arctic Coastal Tundra, Brooks-British Range examined spatial distribution patterns of macrophyte species Tundra, Kalaallit Nunaat High Arctic Tundra, Kalaallit Nunaat composition, alpha diversity (i.e., species richness), and beta Low Arctic Tundra, Northwest TerritoriesTaiga, Scandinavian diversity and its component parts (i.e., turnover and nestedness) Coastal Conifer Forests, and Scandinavian Montane Birch for regions with numerous data records. Using this approach, Forest and Grasslands. Beta diversity was lowest (high inter- we produced a baseline for current macrophyte species station composition overlap) in the remote ecoregions with distribution and composition to which future monitoring results low connectivity, such as the Faroe Islands Boreal Grasslands, can be compared. Knowledge gaps related to macrophyte High Arctic Tundra, and Kola Peninsula Tundra. For most monitoring in lakes and rivers were also identified. ecoregions, turnover was the dominant component of beta 54 A B C D Figure 4-22 Results of circumpolar assessment of lake macrophytes, indicating (a) the location of macrophyte stations, underlain by circumpolar ecoregions; (b) ecoregions with many macrophyte stations, colored on the basis of alpha diversity rarefied to 70 stations; (c) all ecoregions with macrophyte stations, colored on the basis of alpha diversity rarefied to 10 stations; (d) ecoregions with at least two stations in a hydrobasin, colored on the basis of the dominant component of beta diversity (species turnover, nestedness, approximately equal contribution, or no diversity) when averaged across hydrobasins in each ecoregion. diversity as it accounted for more than 70% of the total primarily driven by species turnover, indicating that differences beta diversity (Figure 4-22d). This indicates that variation in among stations were due to the replacement of species. diversity within an ecoregion is due species replacement across stations, rather than finding a subset of the species 4.4.3.3. Compositional Patterns found at the richest station. The High Arctic Tundra ecoregion had no beta diversity as species composition was the same All major taxonomic groups were included in the among stations, and beta diversity of the Kola Peninsula circumpolar dataset, although there were several lakes Tundra was a result of both turnover and nestedness. without macrophytes or with only aquatic mosses. The most common taxa were Myriophyllum alterniflorum, Potamogeton 4.4.3.2. Regional Diversity gramineus, and Ranunculus reptans. Aquatic moss species comprised a higher percentage of total species richness with Species richness of circumpolar macrophytes varied widely increasing latitude. Bryophytes (or charophytes) commonly among lakes in the sub-Arctic region, ranging from 0 to dominate the macrophyte assemblages in high latitude lakes a maximum of 29 species when mosses and algae were (e.g., Welch and Kalff 1974, Vincent and Hobbie 2000) where excluded (Figure 4-23). The highest alpha diversity was macrophyte growth rate is extremely low (e.g., Sand-Jensen observed in Fennoscandia and the Faroe Islands, and alpha et al. 1999). Multivariate analysis of macrophyte assemblages diversity was significantly lower in Greenland (Figure 4-23). for highly-sampled regions indicated some separation Species richness was highly variable in Fennoscandia, owing among countries based on species composition (Figure 4-24). in part to the wide variety of stations and ecoregions sampled In particular, macrophyte species composition in Greenland in that area (Figure 4-23). Beta diversity in these regions was and Norway differed from stations in Sweden and Finland, 55 which were highly similar (Figure 4-24). Species composition 4.4.4. Gaps in Knowledge and Monitoring in a number of Greenland stations was distinct from all other countries included